The overall objective of this research is to identify factors related to the potency and selectivity of immunosuppression induced by rabbit anti-human thymocyte globulin (RATG) in the Rhesus primate by integrated in vivo and in vitro studies. Short term administration of standardized RATG preparations at 50 mg/kg/day (day -1 to day 4 post-transplant) prolongs survival of skin allografts from unrelated MLC incompatible donors to a mean of 32.3 days (range 26-52 days) compared to 8.7 days in controls. Direct lymphocyte mediated cytotoxicity (LMC) was delayed (p less than 0.001) in RATG treated animals and its appearance was accompanied by strong serum blocking activity. Antibody-dependent cell-mediated cytotoxicity peaked at the time of LMC peak in controls, but did not peak until after rejection of the ATG treated recipients. The cytotoxic titer of specific and T cell antibody in RATG was directly correlated with skin allograft survival times (r equals 0.731). The slope of recovery of percent E rosetting cells did not correlate with the rejection, (r equals 0.061) but the recovery of total circulating T cells was predictive of rejection (r equals 0.553). BIBLIOGRAPHIC REFERENCES: Thomas, J., Thomas, F. and Lee, H.M.: Immunological monitoring of long surviving renal transplant recipients. Surgery 81: 125-131, 1977. Thomas, J., Thomas, F. and Lee, H.M.: Studies of lymphocyte-dependent antibody in chronic renal allograft rejection. Transplant. Proc. 9:65-68, 1977.